The invention relates to a weighing cell, more specifically a weighing cell module with a force-transfer mechanism that includes a parallel-guiding linkage with a vertically movable parallelogram leg and a spatially fixed parallelogram leg.
In a weighing cell as the core element of a balance, the force-transfer mechanism—based on its design for a given maximum load and based on the accuracy that it is capable of achieving—essentially determines the range of applications that a balance can be used for.
Commercially available balances are often structured in so-called families or series of balance models, where the balances within a model family have a similar appearance and are identified by similar model designations. A family of balances is often the result of either a single development project or of a group of mutually connected projects.
Balances of the existing state of the art have the problem that within a model family, the respective weighing cells for different maximum loads and different measurement resolutions can differ strongly from each other in regard to their overall design and their subassemblies. Thus, a large number of variations exists among the subassemblies, which leads to high production and inventory costs.
In particular, the different models within a family of balance models also frequently differ in regard to the size of the weighing pan and the way in which the weighing pan is coupled to the weighing cell. According to a known concept that has proven useful for balances of higher load capacities, the weighing pan which in this case normally needs to be of a larger size, or a weighing-pan support if applicable, is coupled at several (in most cases four) points to the load-receiving part of the weighing cell, i.e., the vertically movable parallelogram leg of the force-transfer mechanism, in order to avoid detrimental effects from eccentrically positioned weighing loads. In low-capacity balances on the other hand, which in most cases have a small weighing pan, the preferred arrangement is to couple the weighing pan to the load-receiving part through a so-called single-point connection, for example through a conical support serving as a seat for the weighing pan.
The known state of the art includes balances in which individual components are already configured in a way that allows them to be adapted to different maximum loads in a relatively economical way.
For example, a balance that is disclosed in EP 0 573 806 A1 has a measuring cell that is connected to a U-shaped intermediate support frame through a form-fitting and force-transmitting connection. The measuring cell is arranged between the U-legs of the intermediate support frame and fastened to the base section of the U-frame. The respective contact surfaces on the intermediate support frame and on the measuring cell are finished within very narrow tolerances, so that no assembly stresses are introduced into the measuring cell when the support frame and the measuring cell are bolted together. Thus, the measuring cell can be adjusted together with the intermediate support frame prior to installation in a balance housing, and the measuring cell and support frame can be installed into a housing as a unit. The U-shaped intermediate support frame is designed to receive measuring cells of different widths.
An overload-protection system for a precision balance described in DE 295 14 793 U1 has a secondary parallel-guiding linkage with upper and lower guide arms, where the ends of the arms that face towards the weighing pan are joined to a connecting leg and the ends that face away from the weighing pan are joined to the load receiver, so that the guide arms, the connecting leg, and the load receiver are tied together in the manner of a parallelogram linkage. The overload protection system includes at least one pre-tensioned spring that keeps the weighing pan and the load receiver rigidly coupled to each other within the weighing range of the balance. The spring is positioned between the upper guide arm and a seating plate that is rigidly connected to the load receiver. The connecting leg passes with lateral clearance through the seating plate. With this design concept it is possible to arrange the overload protection system primarily in a lateral position at the front end of the measuring cell so that it takes up little space.
In addition, a receiving device for a calibration weight can be fastened to or integrated in the guide plates that contain the flexure pivots and are connected by two guide bolts, or it can be fastened to or integrated in the seating plate of the secondary parallel-guiding linkage. Thus, the overload protection system can be preassembled outside the balance and adjusted to the maximum load capacity of the balance. This subassembly is connected to the measuring cell through a small number of screws. The device can be adapted to different load ranges by using a spring with a different spring constant.
However, although the devices disclosed in the prior art are designed to use some of the same individual components in more than one balance model, there is still a relatively large diversity in respect to the overall number of subassemblies. Particularly if balances have to be equipped with weighing pans of different sizes, e.g., small or intermediate-sized or large weighing pans, it is necessary to make accommodations in the design for a stable coupling of the differently sized pans to the weighing cell. Thus, weighing pans exceeding a certain size can no longer be held by means of a cone with a single-point connection to the weighing pan, because the effects of an eccentric position of the weighing load could have too large an influence on the weighing result. An overload device, too, has to meet different requirements depending on the size of the weighing pan. The influence of laterally directed torques which can have an effect on the force-transfer mechanism and can ultimately cause its destruction increases with larger sizes of weighing pans. The objective is to intercept these laterally directed torques.